The present invention relates to a method of producing an aluminum material for use as an electronic material, for example, for use as a wiring material for high-density integrated circuits or as a material for bonding wires for connecting electrodes on IC chips to leads on external packages.
"ppb" as used herein is by weight.
Such bonding wires and wiring for LSIs, VLSIs and like high-density ICs are prepared from aluminum materials such as high-purity aluminum, Al-Si alloy, Al-Si-Cu alloy and Al-Cu alloy. Among these aluminum materials, the high-purity aluminum heretofore used is prepared by the three-layer electrolytic purification process. The other aluminum alloys in use are prepared by adding alloy components to the high-purity aluminum obtained by the above process.
However, the three-layer electrolytic purification process involves extreme difficulties in reducing the U and Th contents of aluminum owing to contamination, so that the high-purity aluminum obtained by this process usually inevitably contains about 20 to about 300 ppb of U and Th. Consequently, U and Th are present in the bonding wires and IC wiring which are made of the high-purity aluminum or the aluminum alloy prepared from the high-purity aluminum. A problem is therefore encountered in that the radioactive emissions released form U and Th, especially alpha rays, produce software errors in integrated circuits.
Accordingly, processes have been proposed for reducing the U and Th contents of aluminum materials to diminish such software errors. The first of these processes is a strip melting process (see Unexamined Japanese Patent Publications SHO 61-59761 and 61-59760).
The second of these processes comprises the steps of evaporating the aluminum material to be purified and containing U and Th within a vacuum container, ionizing the evaporated aluminum material, applying a magnetic field to the ionized aluminum material to change the impurity concentration of the material, and separately collecting a portion of the aluminum material having an increased impurity concentration and a portion of the material having a decreased impurity concentration (see Unexamined Japanese Patent Publication SHO 57-164942).
The first process must be practiced in a sustained equilibrated state, requires a long period of time and is industrially very costly. The second process is extremely low in the purification efficiency achieved by one operation as disclosed in the publication, needs to be practiced repeatedly many times and is also industrially very costly.